Household appliances and the like usually have a flexible cord with a terminal plug at the end for connection to a single phase electric outlet. Ordinarily, the cord includes two power conductors and in some cases an additional grounding conductor. In order to prevent injury to persons coming in contact with the appliance it is important that accessible places be electrically isolated from the power busses of the appliance. There are standard tests for measuring the leakage current to ground from electric appliances, and safe leakage current standards have been established and appear in the IEEEE Standard Dictionary of Electrical and Electronics Terms.
In the past, these leakage tests have been performed by connecting the power terminals of the equipment under test to an electric power source through a reversing, double pole, double-throw switch. With the switch in one position an ammeter was connected between earth ground and various accessible locations on the appliance while the appliance was itself insulated from earth ground. The switch was then actuated to the other position and the test was repeated. The leakage test has also been carried out without use of the reversing switch, but by reversing the position of the appliance plug in the electrical outlet. This latter procedure can not be used of course, where the plug includes a grounding pin. When using either of these prior art testing procedures, the person performing the test is subjected to the possibility of electric shock if the appliance under test is dangerously defective.